Far-right French presidential candidate, Marine Le Pen, has attacked her centrist rival Emmanuel Macron, suggesting he is a “hysterical, radical Europeanist” who is weak on jihadi terror.

Her tirade came as the country’s demoralised mainstream parties threw their weight behind her opponent on the first day of campaigning for the presidential runoff on 7 May.

Announcing she was stepping aside temporarily from the presidency of the Front National to be “above partisan considerations” and devote herself to the race for the Elysée, Le Pen said of Macron: “He is for total open borders. He says there is no such thing as French culture. There is not one area where he shows one ounce of patriotism.”

Le Pen finished second to the former investment banker in the first round vote on Sunday. In a symbolic gesture aimed at broadening her appeal and underlining the idea that the president of France should represent the whole country, not just their party, she told France 2 television she was now “no longer the president of the Front National. I am the candidate for the French presidency”.

Politicians from the Socialist and Les Républicains parties – the mainstream centre-left and centre-right groups that have dominated French politics for decades, but found themselves shut out by voters – united on Monday to urge the country to back Macron and reject Le Pen’s populist, anti-EU and anti-immigration nationalism.

The outgoing president, François Hollande, said he would vote for Macron, his former economy minister, because Le Pen represented “both the danger of the isolation of France and of rupture with the EU”. A far-right president would “deeply divide France”, he said. “Faced with such a risk, it is not possible to … take refuge in indifference.”

Only the defeated far-left candidate, Jean-Luc Mélenchon, pointedly refused to endorse Macron, saying he needed to consult his base first.

Le Pen’s aim in temporarily stepping aside from her party’s presidency is to appeal to the supporters of losing first-round candidates, particularly some of those who backed the conservative François Fillon, who finished third, and the minor rightwinger Nicolas Dupont-Aignan.

She had “always considered the president of France as the president of all of the French people, who must unite all of the French people”, she said. The move would allow her more freedom to “change certain aspects of her project and rally support”, party sources told Le Monde.

Macron, 39, who founded his En Marche! movement only this time last year and has never held elected office, became the clear favourite to become France’s youngest president after winning 24.01% of the vote, ahead of Le Pen’s 21.3%. Polls have consistently predicted Macron would win a head-to-head contest between the two by up to 25 points.

Le Pen leapt on the string of endorsements for Macron to paint him as the candidate of a discredited political establishment, attacking the “rotten old republican front” of centre-left and centre-right parties that have aligned to keep the FN from power whenever it has come close to it.

“I’ve come here to start the second round campaign in the only way I know, on the ground with the French people,” she said on a visit to a market in the northern town of Rouvroy. She wanted to draw the French people’s attention to “important subjects including Islamist terrorism, on which Mr Macron is, to say the least, weak,” she said.

France has suffered a series of terror attacks by violent jihadis over the past two years that have left 239 people dead.

On Tuesday both candidates will attend a ceremony honouring a policeman who was shot dead on the the Champs Élysées less than less than 72 hours before Sunday’s vote. Two other officers were seriously injured in the attack, which was claimed by Islamic State.

Macron’s upbeat, internationalist vision of a tolerant France and a stronger, united Europe with open borders is in stark contrast to Le Pen’s French-first policies which aim to suspend the EU’s open border agreement on France’s frontiers, expel foreigners on intelligence service watchlists, restore the franc and possibly leave the EU.

Le Pen, whose father, Jean-Marie, reached the 2002 presidential election runoff, gained 1.2 million new voters compared with her 2012 presidential bid. Louis Aliot, a Front National vice-president, said she offered an alternative for patriots, adding he was “not convinced the French are willing to sign a blank cheque for Mr Macron”.

The euro and European stock markets rose and France’s main share index hit its highest level since early 2008 on Monday as investors bet on Macron becoming president. Relieved politicians including the German chancellor, Angela Merkel, wished him well.

Merkel’s chief of staff, Peter Altmaier, tweeted that the win for Macron showed “France AND Europe can win together! The centre is stronger than the populists think!”

In a highly unusual gesture during an ongoing campaign, the European commission president, Jean-Claude Juncker, also welcomed the first round result. The commission’s spokesman, Margaritis Schinas, said the choice “was a fundamental one” since Macron represented pro-EU values while Le Pen “seeks its destruction”.

The conservative Le Républicains candidate Fillon won just under 20%, marginally ahead of Mélenchon. He was the favourite until January when his campaign was hit by allegations that he had given his British-born wife, Penelope, a fake job as his parliamentary assistant.

The official Socialist party candidate, Benoît Hamon, got just 6%. “We are in a phase of decomposition, demolition, deconstruction,” said the former Socialist prime minister Manuel Valls. “We didn’t do the intellectual, ideological or political work on what the left is and we paid the price.”

Valls pledged his support to Macron, saying: “We must help him as much as we can to ensure Le Pen is kept as low as possible.”

Whichever candidate wins in the second round on 7 May will need to try to build a majority five weeks later in parliamentary elections. The Front National has only two members in the French national assembly and En Marche! none, although Macron has enlisted about 50 sitting Socialists and several centre-right grandees.

Voters in France went to the polls on Sunday in the first round of the presidential election. The top two candidates, independent Emmanuel Macron and far-right Marine Le Pen, go into a run-off in a fortnight. Find out where each drew most support and what happens nextFrench presidential election 2017 – first round live updates